Flexography is a printing process which utilizes a flexible
relief plate that can be adhered to a printing cylinder. It is basically an
updated version of letterpress. It much more versatile than letterpress in that
it can be used for printing on almost any type of substrate including plastic,
metallic films, cellophane, and paper. It is widely used for printing on the
non-porous substrates required for various types of food packaging. It is also
well suited for printing large areas of solid color.

Applications

Flexography continues to be one of the fastest growing print
processes and is no longer reserved just for printing specialty items. The ability
of flexography to print on a variety of substrates allows the process to be
used for a wide range of printed products. Food packaging is an important market
because of the ability of flexography to print on non-porous substrates. This
ability makes it useful for printing on plastic bags as well. Other common applications
printed with flexography include gift wrap, wallcovering, magazines, newspaper
inserts, paperback books, telephone directories, and business forms.

Printing Process

The relief plate used for flexography is made of molded rubber
or photopolymer materials with the image areas raised above the non-image areas
of the plate. Flexographic plates can be created with analog
and digital platemaking
processes.

Flexography is a direct printing method in that the inked plate
applies the image directly to the substrate. An inked roller known as the "anilox
roller", applies ink to the raised portions of the plate which is then
transferred to the substrate. The anilox roller has cells that carry a specific
amount of ink to the plate. The number of cells per linear inch can vary according
to the type of print job and the quality required.

The name "anilox" is derived from the ink that was
used for the process until the 1950's. Anilox ink was manufactured with "aniline"
dyes which, in the 1950's, were discovered to be health hazards, so pigment
based inks were developed and have been used ever since. The ink carrying roller
has continued to be called the "anilox roller" even though the aniline
dye inks are no longer used for flexography. The current inks are very fluid
and dry rapidly and are most often water based.

Press Types

Flexographic printing is accomplished on rotary web presses.
The presses can be divided into three main categories: stack press, in-line
press, and common impression cylinder press.

Stack Press: The stack press has separate
printing units stacked vertically and each printing unit has its own
impression cylinder. This was the first type of press developed for
flexography. It is difficult to hold registration between multiple
colors printed on stretchable substrates using the stack press, even
with web tension controls. It is best suited for heavier gauge substances,
such as thick paper products, that will not stretch or for applications
that do not require good registration.

In-Line Press: Like the stack press, the in-line
press has separate printing units for each color and each unit has
its own impression cylinder, but they are arranged in a horizontal
configuration just like a rotary offset press. Because of the distance
between the printing units, problems with print registration can occur.
Tension control equipment is used on the press to hold proper registration
between multiple colors. The wider in-line presses are used for printing
large items such as corrugated cartons and smaller width webs are
used for printing pressure sensitive labels at high speeds.

Common Impression Cylinder Press: Instead
of the printing units being independent from the others, they are
all grouped around a common impression cylinder. Substrates do not
stretch as they move around the impression cylinder so the common
impression cylinder press is a good choice for printing on substrates
such as thin plastics that would normally stretch when used on other
types of presses. This type of press provides for better registration
of multiple colors. Some CIC presses have impression cylinders that
are as much as 8 feet in diameter which allows for as many as 8 stations
to be placed around the cylinder. The only drawback of the CIC press
is that they can only print on one side of the substrate.